Fix ASKSyntaxViewController actions and correct style

This branch includes the following fixes:

* `ASKSyntaxViewController`'s code style has been updated to match other parts of SyntaxKit.
* `-indentSelection:` and `-unindentSelection:` now fully understand the indentation properties on `ASKSyntaxViewController`.
* `-indentSelection:`, `-unindentSelection:`, and `-toggleCommentForSelection:` now behave more like their Xcode counterparts in the way they apply to whole lines and handle selections.
* Backspacing into the leading whitespace of a line now removes a level of indentation.